Field of the Invention
The present invention relates to a radiation imaging apparatus and a radiation imaging system.
Description of the Related Art
Recently, radiation imaging apparatuses have been devised to minimize the exposure dose by properly managing radiation. For example, there is available a method (AEC (Auto Exposure Control)) of automatically stopping the emission of radiation when, for example, the optimal amount of radiation for diagnosis is applied. It is possible to perform proper diagnosis with the minimum necessary radiation dose by stopping the emission of radiation when a specific region of a patient which is designated by a doctor or radiation technician is irradiated with a predetermined amount of radiation. When performing AEC, it is necessary to properly determine with which region of a patient has been irradiated how much amount of radiation.
Japanese Patent Laid-Open No. 2013-176544 discloses a radiation imaging apparatus which performs auto exposure control. This radiation imaging apparatus divides an irradiation field into a non-object area, an implant area, and an object area based on a histogram. This apparatus then specifies the minimum value pixel and the maximum value pixel in the object area, and stops the emission of radiation based on the integrated pixel values of the minimum value pixel and the maximum value pixel. In this case, a non-object area is defined as an area directly irradiated with radiation without transmission through an object, an implant area is defined as an area where an implant exists in the object, and an object area is defined as an area obtained by excluding the non-object area and the implant area from the irradiation field.
The apparatus disclosed in Japanese Patent Laid-Open No. 2013-176544 divides an irradiation field into a non-object area, an implant area, and an object area based on a histogram. In practice, however, there is an area which should not be classified to either a non-object area or an object area. When taking chest portion imaging as an example, an area having a pixel value which is the second largest to a non-object area is not a lung field area but is the boundary area between the non-object area and the skin of the object. Therefore, on a histogram with one axis representing pixel values and the other axis presenting frequencies in pixel value classes, the boundary area is located between the non-object area and the object area.
The apparatus disclosed in Japanese Patent Laid-Open No. 2013-176544 detects the maximum peak, of the peaks of a histogram, which indicates the maximum pixel value, uses, as a non-object threshold, a pixel value obtained by multiplying the pixel value of the maximum peak by the ratio of pixels located in the negative direction relative to the pixel value, and specifies, as a non-object area, an area exhibiting pixel values equal to or more than the non-object threshold in the irradiation field. In addition, the scheme disclosed in Japanese Patent Laid-Open No. 2013-176544 is designed to stop the emission of radiation based on the integrated pixel value of a maximum value pixel in an object area adjacent to a non-object area on a histogram. However, the maximum value pixel is likely to be a pixel in a boundary area. In such a method, therefore, the accuracy of exposure control is low. The same applies to the determination of a minimum value pixel.
In addition, even if no non-object area exists, exposure control should not be performed by using pixels in a specific range including the maximum value pixel. If no non-object area exists, it can be considered that a maximum value pixel is located at a specific region in an irradiation area. The radiation emitted from a radiation source has a spatial distribution, and radiation having a very high output is sometime generated in a partial area. In this case, there is a possibility that a maximum value pixel is a pixel located at a position different from the position of a region targeted by a doctor or radiation technician.
Furthermore, likewise, in a case in which there is no non-irradiation area such as an implant area, exposure control should not be performed by using pixels in a specific range including the minimum value pixel. Since the pixel value of the minimum value pixel includes a large noise component, the accuracy of exposure control based on the pixel value of the minimum value pixel is low.